According to the American Wood and Forest Association's “Details for Conventional Wood Frame Construction”, wood frame construction continues to be the predominant method of constructing homes and apartments. This is due to the inherent strength and durability of wood frame buildings. Increasingly, wood framing is also being utilized in the construction of commercial and industrial mid-rise buildings. Wood frame buildings are economical to build and to heat and cool down, providing comfort for the occupants. Moreover, wood construction is readily adaptable to a wide variety of architectural building styles.
There are two (2) predominant styles of wood frame construction in the building industry: balloon and platform (see, e.g., FIGS. 1, 2, 3 and 4). In general, balloon framing is a technique that suspends the floors from the walls. Vertical wood studs extend the full height of the walls of a balloon frame building, and floor joists are fastened to the studs with nails. Balloon construction is a system of framing a wooden building, whereby all vertical structural elements of the exterior bearing walls and partitions consist of single studs that extend the full height of the frame, from the top of the sill plate to the roof plate, and all floor joists fasten by nails to the studs.
The balloon-frame house with wood cladding, invented in Chicago in the 1840s, aided the rapid settlement of the western U.S. The introduction and ensuing popularity of balloon frame construction coincided with the intensification of the settlement of Wisconsin and the opening of Wisconsin's forests to the lumber industry. By 1892, the vast amount of milled lumber available made balloon frame construction an inexpensive and expedient choice for Wisconsin builders, and wood frame buildings of all descriptions became ubiquitous on the landscape. This method of construction was common until the late 1940s.
The balloon style of construction has mostly been discontinued due to a number of factors, including, but not limited to the overall low fire resistance and the high cost of lengthy studs, which together inhibits the use of the balloon method of construction in multi-story buildings. This led the industry to the platform style of construction, in which each floor of the building is built as a separate unit from floors above and below it. In North America, with its abundant softwood forests, the framed building received an extensive revival after World War II in the form of platform framing. Since that time, platform framing has become the predominant form of wood frame construction.
In a contemporary multi-story building, a general platform construction sequence can be briefly described as follows. Reference is made to FIGS. 1, 2, 3 and 4. FIG. 1 shows a typical section view cut through the exterior bearing wall, where the floor framing is perpendicular to the exterior wall. This typical section (FIG. 1) pertains to a platform construction and illustrates a typical floor joist bearing over the bearing one-sided shear wall below with an upper story bearing shear wall above, conceptually showing typical/standard gravity and lateral load transfer connections, structural floor diaphragm and other major non-structural elements, such as exterior stucco, drywall and flooring, etc.
FIG. 2 shows a typical section view cut through the interior bearing wall where the regular wood joist floor framing on the left side is parallel to the subject wall below and above and the wood framing on the right hand side is perpendicular to the interior wall above and below. This typical section pertains to platform construction and it illustrates a typical floor joist bearing over the bearing one-sided shear wall below with an upper story bearing shear wall above, conceptually showing typical/standard gravity and lateral load transfer connections, structural floor diaphragm and other major non-structural elements (such as exterior stucco, drywall and flooring, etc.).
Upon completion of the earthwork (i.e., excavation for the foundation), a foundation is typically laid and installed. Thereafter, first floor walls are erected, ending with a double top plate 4 on top of the studs 1. Then, the floor framing elements, such as floor joists 3 and blocking 7, or floor joists 26 and blocking 28 and 29 if an engineered wood framing system is utilized, are added. The subfloor plywood 11 is then constructed. Subfloor 11 is generally defined in the construction industry as “rough” floor, typically plywood, over which flooring material 18 is laid. Subfloor membrane 11 is attached to the floor framing system below with fasteners 24 in accordance with the floor diaphragm fastening schedule, forming a structural floor diaphragm that is defined and discussed in a greater detail below. After the second floor base plate 8 is installed over the subfloor, the wall studs 2 go up to the third-floor level to a top plate again. Over that top plate, the process is repeated for the next floor up and so forth. The ceiling structure at the roof level and the rood structure itself are installed over the very last double top plate. Once rough framing of the structure is complete (i.e., the structure skeleton is erected), including but not limited to the installation of shear transfer hardware 6 and 9, then other non-structural elements of the building, such as but not limited to exterior stucco 12, exterior building paper and wire mesh 13, interior drywall sheathing 14, wall thermal insulation 15, floor thermal insulation 16, and interior drywall ceiling sheathing 23 are scheduled for installation, traditionally postponing the installation of flooring material 18 towards the very end of the structure construction sequence.
The advent of contemporary construction technologies brought engineered wood to the construction industry market as an alternative material choice to the traditional wood. Engineered wood products, see FIGS. 3 and 4, are typically used in a host of structural applications, ranging from home construction to agricultural buildings to large commercial structures.
FIG. 3 shows a typical section view cut through the exterior bearing wall where an engineered wood I-beam floor framing is perpendicular to the exterior wall. This typical section pertains to platform construction and it illustrates a typical engineered wood I-beam floor joist bearing over the bearing one-sided shear wall below with an upper story bearing shear wall above, conceptually showing typical gravity and lateral load transfer connections, structural floor diaphragm and other major non-structural elements, such as exterior stucco, drywall and flooring, etc.
FIG. 4 shows a typical section view cut through the interior bearing wall where an engineered wood I-beam floor joist framing on the left side is parallel to the subject wall below and above and an engineered wood I-beam floor framing on the right hand side is perpendicular to the interior wall(s) above and below. This typical section pertains to platform construction and it illustrates a typical engineered wood I-beam bearing over the bearing one-sided shear wall below with an upper story bearing shear wall above, conceptually showing typical gravity and lateral load transfer connections, a structural floor diaphragm membrane, and other major non-structural elements such as exterior stucco, drywall and flooring, etc.
In both residential and commercial construction, engineered wood products are typically used in longer span floors with reduced or limited deflection criteria, walls, and roofs. Use of engineered wood applications have not introduced any principal changes to the normal platform construction sequence briefly described above.
Blocking noise from floor-to-floor is the most common, yet challenging request in soundproofing. While a lack of a desired level of floor sound suppression persists in the construction industry, the current industry interpretation of the term “sound barrier” refers to a system that decreases propagation of sound traveling through the floor system. Regretfully, sound suppression continues to play role of a sound or noise propagation control rather than a sound barrier system.
Rick Berg's article “Using a Sound Barrier With Wood Flooring” in the June/July 2002 edition of Hardwood Floors Magazine recognizes significant ongoing customer demand for a “. . . better job of controlling sound transmission between living quarters,” noting that building codes typically specify two types of sound-control ratings: IIC (Impact Insulation Class) and STC (Sound Transmission Class). A rating of 50 decibels for each class is generally is a standard requirement. The IIC class relates to sound transmitted as a result of impact on a surface, such as footsteps on a floor for example. The STC class relates to airborne sounds, such as voices and music. Sound control underlayments often carry an STC rating, as well as an IIC rating. However, flooring products really have a substantial effect only on impact sounds.
The aforementioned article reveals that “in some cases, we've seen developers asking for a IIC in the 60s. . . . Sometimes you can achieve that in a concrete structure with suspended ceilings, but you can't expect to be in the 60s with a wood-frame structure. The structure itself limits that.” In reality, a rating in the range of 50 decibels or even 60 decibels for wood frame structures is well below the desired range of high 80 decibels or even 90 decibels. Current art pertinent to the acoustic materials in the industry include materials for sound insulation in wood frame construction that typically rely on employing of one (1) or more types of noise propagation reduction systems from the following general list:
1. Use of actual flooring materials as soundproof material. Obviously, and as said in the aforementioned article, different flooring materials have very different sound transfer qualities. Carpet flooring, for an example, is a material with one of the highest soundproof ratings. However, it is highly problematic due to a number of factors, including, but not limited to, the major known issues of indoor air quality, and serviceability issues associated with particle residue retained between the carpet pad and carpet itself. Such residue is known to cause allergies, breathing problems, respiratory infections and asthma. Furthermore, accumulation of moisture and, as a consequence, most likely growing bacteria such as mold that is not removable by means of regular cleaning, creates a major problem for the consumers, not to mention the overall high maintenance factor.
2. Use of sound control underlayment, such as cork or even an engineered noise control insulation mat that is intended to limit only a certain percentage of impact noise between the floors. If sound control underlayment is employed, it is normally installed between the flooring 18 and plywood sheathing 11 (refer to FIGS. 1 to 4). Sound control underlayment is not called out in FIGS. 1 to 4 since it does not embody the industry standard or mandatory requirement in all the typical cases.
3. Interior drywall sheathing 23 per FIGS. 1 to 4 or, in older construction, use of so called acoustic ceiling, also known in the industry as “popcorn ceiling” instead of drywall sheathing 23. The “popcorn ceiling” can be found in some of the older structures since it was popular from the late 1950's through the early 1980's. Even if difficulty in cleaning and the issue of architectural appearance are negated and not considered as main factors against use of acoustic ceilings, the main prohibiting factor against this type of ceiling today is the presence of asbestos.
Interior drywall sheathing 23 itself is not very effective as a primary sound reduction system. Some local building and safety jurisdictions suggest addition of ⅝ inch gypsum board to the existing ceiling construction, while other jurisdictions, depending on building occupancy and other factors beyond the scope of this discussion, simply require doubling drywall sheathing 23 to achieve a satisfactory reduction in noise propagation. In either case, even a 0.5 inch thickness increase in ceiling board system essentially means an increase on the overall dead load of the floor system by 2.5 pounds per square foot. Obviously, such an approach offers a less than desirable solution from both the design gravity load standpoint and the design lateral load increase standpoint. Meanwhile, all of the systems described above offer a noise transmission reduction remedial solution that operate in the 50 decibel range or at the very best 60 decibel range.
Although the acoustic engineering society has made attempts in the past to work on finding a solution in form of an improvement in the current state of the art, the building community has created an opposition that has thus far blocked these attempts due to the increase in the cost of construction. However, a lack of a proper noise blocking barrier can lead to medical problems associated with exposure to noise. Complications, related to the exposure to certain levels of noise in different environments, may result in an undesirable outcome. For an example, exposure to noise in the hospital or at school is a nuisance that inflicts various negative impacts on patient's and student's nervous system.
Currently, the industry has not yet offered to the consumer a floor-to-floor noise blocking barrier that can operate in the high 80s decibel range or even 90 decibel range, despite the tendency toward higher population densities in urban areas. Privacy at home has become of greater importance, not to mention the rapidly developing trend of multi-level housing that brings the neighbor noise issue to the forefront, highlighting a need for exceptional, non-remedial solutions in form of an adequate noise blocking barrier.
In structural engineering, a diaphragm is generally defined as structural system used to transfer lateral loads to shear walls or frames primarily through in-plane shear stress. These lateral loads are usually the result of wind and earthquake loads, but other lateral loads such as lateral earth pressure or hydrostatic pressure can also be resisted by diaphragm action. Diaphragms are usually constructed of plywood or oriented strand board in timber construction, metal deck or composite metal deck in steel construction, or a concrete slab in concrete construction.
The Second Edition of Dictionary of Architecture & Construction by Cyril Harris defines a diaphragm as “A floor slab, metal wall panel, roof panel, or the like, having a sufficiently large in-plane shear stiffness and sufficient strength to transmit horizontal forces to resisting systems.”
The diaphragm of a structure often does double duty as the floor system and roof system of a building, or the deck of a bridge, which simultaneously supports gravity loads. The common floor diaphragm serves a dual purpose by supporting vertical forces (from loads such as furniture, people, snow, uplift, and its own dead load) and by transmitting horizontal forces (from wind pressure or earthquake accelerations) to the vertical load resisting elements of the structure, such as the shears walls. In the wood frame structure, shear walls play the role of lateral support during the lateral load transfer action. In a common form of sheathed construction, the diaphragm membrane is usually a planar system of sheathing connected to the frame members, intended to act together to withstand considerable in-plane forces. Diaphragm stiffness is an important parameter in the design of wood framed structures to calculate the predicted deflection, and thereby determine if a diaphragm may be classified as rigid or flexible. The two primary types of diaphragms are identified in the industry as flexible and rigid. This classification controls the method by which load is transferred from the diaphragm to the supporting structure below. Flexible diaphragms resist lateral forces depending on the tributary area, irrespective of the flexibility of the members to which they are transferring force. On the other hand, rigid diaphragms transfer load to frames or shear walls depending on their flexibility and their location in the structure.
Parts of a diaphragm include: the membrane, used as a shear panel to carry in-plane shear; the drag strut member, used to transfer the load to the shear walls or frames; and the chord, used to resist the tension and compression forces that develop in the diaphragm, since the membrane is usually incapable of handling these loads alone.
According to the “HISTORY OF YARD LUMBER SIZE STANDARDS” by L. W. SMITH, Wood Technologist and L. W. WOOD, Engineer (Forest Service, U.S. Department of Agriculture), early standards called for green rough lumber to be of full nominal dimension when dry, but the requirements have changed over time. For example, in 1910, a typical finished 1-inch (25 mm) board was 13/16 inch (21 mm). In 1928, that dimension was reduced by 4%, and yet again by 4% in 1956. In 1961, at a meeting in Scottsdale, Ariz., the Committee on Grade Simplification and Standardization agreed to what is now the current U.S. standard: in part, the dressed size of a 1 inch (nominal) board is fixed at ¾ inch; while the dressed size of a 2 inch (nominal) lumber was reduced from 1⅝ inch to the today's standard of 1½ inch. Therefore, currently, typical 2× joist 3 is actually 1.5 inches thick.
More often use of the open space or open floor design concept in contemporary architectural designs require wood floor diaphragms to span farther and farther horizontally without a support (walls, column, etc.). In many cases, architectural design parameters create situations where walls above a floor are not aligned with or not located directly beneath the walls on that floor, thereby requiring certain parts of the floor diaphragm to be responsible for the lateral load transfer from walls above down to the walls below through the floor diaphragm. This situation automatically leads to development of higher stresses within the horizontal diaphragm. The same and/or similar challenges are described in the SEAOSC's article “Thinking Outside the Box: New approaches to very large flexible diaphragms” by John W. Lawson, SE of Kramer & Lawson, Inc. (Tustin, California). However, the aforementioned article notes that “wood roof diaphragms are being required to span farther horizontally with higher shear stresses.”
It is certainly understood that especially high span, flexible wood diaphragm behavior is somewhat similar to the behavior of a beam subjected to bending (flexure). A horizontal wood diaphragm span between vertical supports, for example shear walls in the out-of-plane direction, as schematically shown on FIG. 6. Because of the beam-like behavior in the out-of-plane direction as schematically demonstrated in FIG. 6, lateral force 20 application throughout the diaphragm system causes a different type of stresses to occur within the different components of the diaphragm.
Besides the lateral forces (caused by earthquake, strong wind, etc.) that travel through the diaphragm and cause shear stresses, due to the beam-like behavior in the out-of-plane direction diaphragm, there are also forces or force components that occur in the membrane of the diaphragm and act in direction 49 as shown on FIG. 7 (also FIGS. 5A and 5C), imposing forces onto the plywood panels 11, perpendicular to the direction of the edge spacings 22 that run parallel to (or along) the direction of floor joist 3 or 26. Subject forces 49 imposed in the direction as shown in FIGS. 5B and 5D pull the plywood away, imposing forces in the same direction onto the fasteners 24. Force 49 is also perpendicular to the direction of lateral force 20 and, correspondingly, reaction (and shear transfer) force 41. This action, development and corresponding imposition of a sufficient amount of force in the direction 49 will cause excessive stresses in: (1) the most vulnerable area from the structural point region 37 of wood panel 11 on FIG. 5A and, correspondingly, region 41 on FIG. 5C; (2) fastener 22 on FIG. 5A and FIG. 5D; and (3) joist 3 of FIG. 5A or joist 26 on FIG. 5C, causing cracking or splitting 50 as schematically shown on FIG. 5B and FIG. 5D.
The issue (1) above can also occur if fasteners 24 are located too close to the edge of plywood panels. For a regular construction assembly where 2× framing such as 3 is used, based on the dimension 36 and 22, the dimension 37 would be approximately within one quarter inch. That is in the best case scenario, neglecting normal intolerances associated with field installation that happens routinely. The dimension 40 of FIG. 5C per current standards varies from 1¾ inch for TJI 110 joists to 3½ inches for TJI 560 joists. The heavier the joist 26, the longer the joist span and, correspondingly, the heavier the resulting diaphragm loads. This leads to the introduction of staggered fasteners, spaced closer when dimension 40 jumps to values higher than 1¾ inch. A staggered nailing pattern again leaves the same problem unresolved for at least 50% of the fasteners, located closest to the edge of panel 11, not offering much higher number than 37 on FIG. 5A, and fasteners 24 are still too close to the edge of plywood panels 11. Therefore, it is evident that the problem of fasteners 24 being too close to the edge of plywood panel 11 exist in both cases. This issue of fasteners 24 located too close to the edge of plywood panels 11 in this type of construction leaves an automatic failure path for plywood to tear through the nails and pull away, as shown on FIG. 5B and 5D.
Issue (2) is likely to result in an overstressing in fastener 22 to the point of loss of structural integrity and corresponding flexure (bending), as schematically shown on FIG. 5B and FIG. 5D. Issue (3) above shall be described as crack or split (separation) development 50 as schematically shown on FIG. 5B and FIG. 5D due to localized stress occurrence, caused by the force exerted by each fastener 24 in the row onto the joist 3 on FIG. 5A or joist 26 on FIG. 5C, in the direction of force 49, perpendicular to the wood grain as shown.
As also discussed in the aforementioned SEAOSC's article, a proposed remedy for issue (1) would be the “multiple lines of nails, on 3× and 3× framing, with special inspection.” In addition, the following statement is made in the article: “As in all wood diaphragms, closely spaced nails that align with the wood grain could cause wood splitting that compromises the nail's gripping strength. The use of a staggered nailing pattern and wider framing members minimizes the risk of lumber splitting due to tight nail spacings.” The subject statement reflects one current solution for both roof and horizontal floor diaphragm construction.
The industry standard 4 foot by 8 foot plywood panels 11 are to be typically installed in the wood diaphragm construction in the transverse direction (perpendicular) to the direction of floor joist. Panels 11 are typically staggered and edge spacing lines between plywood panels are thereby normally spaced every 4 feet apart. The aforementioned remedial solution suggests use of 3× or 4× framing at least every 4 feet where panel joints 22 occur. If framing joists are spaced at 16 inches on center, then every third member would be a 4× or 3× wood beam instead of the 2× joist. This offers an almost cost prohibitive, less than practical solution that also increases the dead load of the structure, inadvertently causing an increase in the design seismic load. Higher mass of the structure (dead load) simply means higher seismic load. The natural difference in stiffness between the typical 2× joist and a 4× or 3× wood beam used as a joist in case of uniform long floor diaphragm may also invite issues with uneven gravity load distribution and transfer within the floor system, posting unexpected potential issues with overall floor system long term performance. Obviously, use of 4× or 3× wood beams do not offer an acceptable solution for the issues (1), (2) and (3) above.
As also mentioned earlier, flooring material is traditionally not a part of the structural system of typical wood frame building. Normally, flooring material is not accounted for by the building designers to structurally resist gravity or lateral loads. From a structural standpoint, flooring self-weight or dead load is simply an additional mass to be considered for the gravity and lateral load design of the floor system as part of the structure and, consequently, design of corresponding portions of the structure responsible for carrying and resisting extra loading exerted by this mass.
The average life expectancy of a regular wood structure is in the neighborhood of one hundred years, depending on a number of factors. Throughout the life of the structure, it is usually expected that flooring will be changed periodically. Frequency of removal and replacement with new flooring normally depends on the type and overall serviceability and durability of the flooring material. Traditionally, flooring material in the industry is not used as part of the structural system of the building, often, carpet flooring is installed temporarily, solely to expedite the escrow closure process during the property acquisition and/or in efforts to obtain a formal certificate of occupancy in the new or remodeled building.
Not utilizing flooring as part of the structural system of the building traditionally creates challenges in the industry, including, but not limited to, moot points during the design phase. The structure is designed to carry a certain weight. Whether the structure is designed to carry 1 pound per square foot or 15 pounds per square foot weight of the floor makes a major difference. Often times, not being able to define and, therefore, not knowing the weight of the flooring material while the architectural design decisions related to the flooring choice has not been made or is being changed numerous times during the design process inserts a definiteness issue between the offices of the architect and the engineer. It is the engineer who is simultaneously estimating the structural design of the building, often times not knowing and only assuming a certain weight of the flooring material. This negatively affects both cost of the design and cost of the project during the construction phase. Conservative design for an additional weight may not always represent the safest and most economical design.
To summarize, putting aside the aforementioned challenges that transpire during the design phase due to lack of knowledge of the material weight while designing the actual structure, not utilizing flooring material as part of the structure creates a situation in the industry where flooring material is an afterthought that constitutes merely a burden to the structure of the building, an additional or added extra weight to be carried from the gravity load and lateral load transfer standpoints, without any participation in load resistance.
Strengthening or seismic rehabilitation of the diaphragms in the existing structures as part of the overall seismic strengthening program for the existing buildings is an important development in the current building industry that presents additional challenges. Reference is made to the Chapter 22 of “Diaphragm Rehabilitation Technique” of FEMA 547, and “Techniques for the Seismic Rehabilitation of Existing Buildings”. Although the aforementioned document also states that “Diaphragm failures are less commonly observed in earthquakes,” the same document reveals a significant problem related to “the disruption caused by strengthening the diaphragm [that] can be quite significant, so diaphragm rehabilitation is less commonly employed than adding global strength and stiffness, or improving connection paths.”
In general, FEMA 547 calls inadequate diaphragm strength and/or stiffness as a main deficiency to be addressed by FEMA's rehabilitation technique. FEMA 547 refers to the addition of new wood structural panel sheathing as the “traditional and common approach to diaphragm strengthening,” also stating that “adding fastening and blocking to existing wood structural panel sheathing can also be done.” Furthermore, FEMA 547 on page 22-1 specifically calls for and describes the following proposed techniques:
I. Replacing existing sheathing with new wood structural panel sheathing.
II. Wood structural panel sheathing overlays with new blocking
III. Wood structural panel sheathing overlays without new blocking
Although FEMA 547 addresses the existing wood structural panel diaphragm related issues, mentioning that “an issue that often arises is whether existing joists, which are typically thicker than the code assumed 1½″, can count as 3× blocking. Some engineers ratio values between 2× and 3× code capacities . . . ” The specific problem, associated with stresses caused by the force 49 (see FIGS. 5A, 5B and 6), that occur in the existing wood structural panel diaphragm as described above, is not mentioned.
Another problem related to the use of the proposed remedies by FEMA such as the wood structural panel sheathing overlay technique(s) is the imposition of permanent weight (dead load) onto the existing structural system that may be incapable of carrying such additional dead load without strengthening and/or structural alterations. Although FEMA 547 states that “adding structural wood panel sheathing over existing sheathing adds weight to diaphragm . . . this rarely poses a problem,” it is said thereafter that “ the engineer should consider the issue.” Since plywood weight is equal to 3 pounds per square foot per inch of thickness, even the addition of a ⅝ inch thick plywood panel overlay will cause a permanent increase in the dead load by at least 2 pounds per square foot. Without analysis of the existing structure and possible strengthening of the gravity load resisting system of the existing structure, such an increase in dead load creates an additional burden in form of the overstress, excessive deflections, or in some rare cases even a so called near failure state situation within the existing gravity load resisting system that exists in the older buildings.
Inasmuch as there has been worldwide attempts to develop conceptually new earthquake resisting systems for the buildings, such attempts are mainly focused on vertical earthquake resisting elements. The floor diaphragm as part of the structural earthquake resisting system attracts less attention than vertical earthquake resisting elements, such as shear walls, moment resisting frames, braced frames, etc.